Landlord not returning security deposit

Me and my roommates in Boston, cleaned and left the apartment in a way that landlord required us. There were no damages made to the apartment other than usual wear and tear. Before moving out date, landlord provided us with the to do list, which notified us to clean apartment thoroughly, which we did. Nevertheless we received only 30% of the security deposit. Landlord said that there was furniture left outside of building and they incurred additional costs for that. We complained and explained that we did not leave anything outside the building and that there were dozen of other people moving out from nearby buildings, who could have left it. In addition we were not informed that it was our responsibility to clean the outside area. Landlord does not agree to return the deposit still, what can we do? Thank you.